Because people suck.
Something happens, the other car, who has 2 witnesses in your car say you were at fault. You are by yourself in the car and it's your word against 2 other people. You lose.
Now imagine that you have a dash cam and got the whole thing on video. You send the video to your insurance company, they clear you of all liability and you get your deposit back.
In my case, I got hit when a semi ran a light. I was going straight, he was turning left in front of me coming the opposite way. I had the light, he had the yield. He went anyway, I braked/swerved, he never slowed down and nailed the aide of my car going about 10ish mph. 8k worth of damage.
Fast forward: he has a camera on his car, I was riding alone. Other insurance company tries to declare me 100% at fault. I send in my copy, my company says the other driver is at fault. Goes to arbitration, officially declares the truck driver at fault.
So yeah... As I said... People suck. It's good to have an undisputable witness in the car with you. That's all it really is. The issue nowadays is people don't have a sense of honesty anymore. If they cam save their own backs at the expense of someone else, they will